Report Parameters Probloms

Hello

I'm trying to pass parameters to a .rdlc report with no success using this code:

Dim Report_Param(0)As ReportParameter

Dim aAsString ="A"

Report_Param(0) =New ReportParameter("A", a)

ReportViewer1.LocalReport.SetParameters(Report_Param)

ReportViewer1.LocalReport.Refresh()

The exception is generated in the SetParameters command.

Does anybody know what is the problem?